A number of different medical therapies involve the placement of plural electrodes against tissue located in the body. One such therapy in which positioning of such electrodes is sometimes performed is pain management therapy. Pain management therapy may be necessary when an individual, due to injury or disease, is in a chronic pain state. If little can be done about the source of the pain, an individual may have to live with the chronic feeling of pain. Pain management therapy may also be appropriate when an individual suffers a naturopathic injury or illness. An example of such an injury is when nerve impingement results in nerve damage. A damaged nerve may be the source of pain signals perceived as an ongoing trauma to a region of the body distal to the nerve. A nerve injury may be perceived as a burning sensation.
Pain signals are transferred through the individual's nervous system. More particularly, a pain signal is typically generated by a nerve associated with the tissue that is subject to injury, inflammation or other pain-causing event. The pain signal is transmitted from this local nerve to a network of neurons residing in the spinal cord. The neurons internal to the spinal cord function as the path through which the pain signal is then transmitted to a specific region of the brain. The receipt of the signal by the brain is perceived by the individual as an indication that a particular tissue or organ is in some sort of distress.
Some pain management therapies involve the use of drugs. Certain drugs, for example, interrupt the transmission of the pain signals to the brain. Unfortunately, many of these drugs are globally transmitted through the body via the circulatory system. As a consequence, a drug can produce undesirable systemic effects that may impede the ability of the individual to perform tasks that require constant neural input; driving or machinery operation for example. Some drugs can be administered to a specific site on the body. These drugs only interrupt the transmission of pain signals from a specific location. This type of therapy offers advantages over therapy in which a drug is introduced throughout the body.
To maintain pain relief over a long time period, site-specific drugs are often administrated through portable devices that a patient can wear. A disadvantage of this type of therapy is that an individual may find it difficult to wear this type of device for an extended period of time. It is also known to administer a site-specific drug using a pump implanted in the patient. While these pumps are useful, there are some difficulties associated in refilling them.
An alternative to these therapies is to apply electrical signals to the neural network to counteract the chronic transmission of pain signals. One such therapy involves applying these signals from the epidural space through the dura, the outer covering of the spinal cord, to the nerves forming the dorsal columns of the spinal cord. These columns are located on the dorsal side of the spinal cord. The signals are applied to this portion of the spinal cord because both studies and practice show that, when the nerves in this portion of the spinal cord are stimulated with electrical signals, there is an appreciable masking of the perception of pain.
An advantage of using this electrical stimulus to mask pain signals is that it can be site specific. The control of signals using this type of therapy can often be adjusted so that it does not interfere with the reception or transmission of other neurological signals to or from the brain. Consequently, the masking of the pain signals using this type of therapy does not, for many individuals, impede their abilities to perform daily living tasks.
A number of different electrode array assemblies have been developed that are designed for implantation against the spinal dura. A typical assembly includes multiple individual spaced apart electrodes. Some of these assemblies include a substrate that supports an array of plural rows and columns of individual electrodes. The electrode assembly itself includes a number of longitudinally spaced apart electrodes. Once the electrode assembly is positioned adjacent the dura, current pulses are applied between selected sets of electrodes. These current pulses flow, in part, through the spinal cord. The electrode current flow patterns are experimented with until the individual reports, instead of pain, a more tolerable tingling sensation. This tingling sensation is known as paresthesia.
The Applicants' Assignee's PCT Pat. App. No. PCT/US2009/033769 filed 11 Feb. 2009, the contents of which is explicitly incorporated herein by reference, and are contained in US Pat. Pub. No. US 2011/0077660, also incorporated herein by reference, discloses one electrode array assembly with plural rows and columns of electrodes. The assembly of this invention includes a substrate formed from superelastic material. An advantage of this assembly is that it can be folded into an insertion cannula that is smaller in width than the width of the array itself. The cannula is inserted percutaneously through the skin between vertebra and in the epidural space above the dura. Upon deployment from the cannula, the electrode array assembly unfolds over the dura. An advantage of this invention is that it eliminates the need to make a major incision in the patient and perform a laminectomy or a laminotomy to position the electrode array assembly over the dura.
Electrical stimulation of neural tissue to foster pain relief, sometimes called neuromodulation, is an effective therapy. Nevertheless, like all therapies, success is not guaranteed. Accordingly, presently it is a common practice to initially implant a test electrode array assembly in a patient prior to the implantation of a permanent electrode array assembly. The test electrode array assembly is in the form of a cylinder. After the test assembly is implanted, signals are sourced to the electrodes on the assembly from a source through conductors on the assembly that project out of the patient. The patient's response to the flow of current between the electrodes of the test assembly is monitored, often for several days. Owing to its small size and circular shape, once this analysis is performed, the test assembly can be removed relatively easily.
Ideally, the test results indicate there is a significant likelihood that the patient will benefit from the implantation of a permanent electrode array assembly. However, there is always a chance that the patient did not obtain appreciable relief when the test assembly was activated. This information indicates to the patient and the practitioner that patient may not gain relief if a permanent electrode array assembly is implanted against his/her dura.
The above process reduces the likelihood that an individual is subjected to the expense, and trauma of having a permanent electrode array assembly and complementary pulse generator implanted when there is not a significant likelihood the procedure will offer significant relief.
However, in the event activation of the test array indicates that the patient could benefit from the permanent electrode array, the patient must then be subjected to the trauma of a second procedure to implant the array.
Further, it should be understood that, when the permanent electrode array is implanted, there is no guarantee that its electrodes will be in the same location at which the electrodes of the test assembly were located. Consequently, even though the results with the test array indicated the patient would benefit from such therapy, the results, due to the placement of the permanent array, may yield less than desirable results.